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Abstract:

Embodiments of the present invention generally relate to methods for
cleaning a substrate prior to a deposition process. The methods generally
include multiple cleaning solutions for removing contaminants from a
surface of a substrate. The multiple solutions generally have different
compositions, and each of the solutions contain one or more additives
selected to remove a variety of contaminants. Mechanical agitation may
also be utilized to remove contaminants from the surface of a substrate.
After cleaning a substrate, a material may be deposited on the substrate
surface.

Claims:

1. A method of processing a substrate having a transparent conductive
oxide disposed thereon, comprising: exposing the substrate to a first
cleaning solution comprising hydrogen peroxide and ammonium citrate;
exposing the substrate to a second cleaning solution having a pH within a
range from about 6 to about 7, the second cleaning solution different
than the first cleaning solution; agitating the second cleaning solution;
and depositing a silicon-containing film on the transparent conductive
oxide.

2. The method of claim 1, further comprising scribing the surface of the
substrate with a laser prior to the exposing the substrate to a second
cleaning solution.

3. The method of claim 2, wherein the agitating the second cleaning
solution comprises removing particles of transparent conductive oxide
from a surface of the substrate.

4. The method of claim 3, wherein the agitating the second cleaning
solution utilizes an ultrasonic mixer.

5. The method of claim 4, wherein the second cleaning solution contains
one or more compounds selected from the group consisting of potassium
hydroxide, ammonium hydroxide, and hydrochloric acid.

6. The method of claim 1, further comprising exposing the substrate to a
first rinse solution after the exposing the substrate to a first cleaning
solution and before the exposing the substrate to a second cleaning
solution.

7. The method of claim 6, wherein the rinse solution is deionized water.

8. The method of claim 7, further comprising exposing the substrate a
second rinse solution after agitating the second cleaning solution.

9. A method of processing a substrate having a transparent conductive
oxide disposed thereon, comprising: exposing the substrate to a first
cleaning solution comprising hydrogen peroxide; agitating the first
cleaning solution; rinsing the substrate with deionized water; exposing
the substrate to a second cleaning solution comprising ammonium citrate
or ammonium acetate; and depositing a silicon-containing film on the
transparent conductive oxide.

10. The method of claim 9, wherein the ammonium citrate or ammonium
acetate is present in the second cleaning solution at a concentration of
about 0.1 percent to about 10 percent by volume.

11. The method of claim 9, wherein the hydrogen peroxide is present in
the first cleaning solution at a concentration within a range of about
0.1 percent to about 10 percent by volume.

12. The method of claim 11, wherein the hydrogen peroxide is present in
the first cleaning solution at a concentration within a range of about
0.1 percent to about 5 percent by volume.

13. The method of claim 9, further comprising agitating the second
cleaning solution.

14. A method of processing a substrate having a transparent conductive
oxide disposed thereon, comprising: exposing the substrate to a first
cleaning solution having a pH within a range from about 6 to about 7;
agitating the first cleaning solution; rinsing the substrate with
deionized water after the exposing the substrate to the first cleaning
solution; exposing the substrate to a second cleaning solution comprising
hydrogen peroxide; rinsing the substrate with deionized water after the
exposing the substrate to the second cleaning solution; and depositing a
silicon-containing film on the transparent conductive oxide.

15. The method of claim 14, wherein the hydrogen peroxide is present in
the second cleaning solution at a concentration within a range of about
0.1 percent to about 5 percent by volume.

16. The method of claim 14, further comprising rinsing the substrate with
deionized water prior to the exposing the substrate to a first cleaning
solution.

17. The method of claim 14, further comprising agitating the second
cleaning solution after the exposing the substrate to a second cleaning
solution.

18. The method of claim 17, wherein the agitating the second cleaning
solution utilizes an ultrasonic mixer.

[0005] In the production of photovoltaic cells and modules, such as
amorphous silicon and microcrystalline solar cells, the efficiency of the
individual cells can be an important industrial concern. One factor that
may affect the efficiency of the photovoltaic cells is foreign matter or
contaminants present during film processing. Contaminants present on the
surface of the cell prior to or during film processing can reduce the
overall conversion efficiency of the solar cell, and may lead to the
formation of hot spots in assembled photovoltaic modules. Hot spots may
occur when one of the photovoltaic cells in a photovoltaic module are
electrically mismatched compared to the rest of the cells in the
photovoltaic module. The presence of hot spots generates localized heat
which may lead to device failure. However, improving the efficiency of
each photovoltaic cell reduces the electrical mismatch between
photovoltaic cells, thus reducing the occurrence of hot spots.

[0006] Therefore, there is a need for improved methods of manufacturing
photovoltaic cells and modules with reduced contamination and higher
conversation efficiency.

SUMMARY OF THE INVENTION

[0007] Embodiments of the present invention generally relate to methods
for cleaning a substrate prior to a deposition process. The methods
generally include exposing a substrate to multiple cleaning solutions to
remove contaminants from a surface of a substrate. The multiple solutions
generally have different compositions, and each of the solutions contain
one or more additives selected to remove a variety of contaminants.
Mechanical agitation may also be utilized to remove contaminants from the
surface of a substrate. After cleaning a substrate, a material may be
deposited on the substrate surface.

[0008] In one embodiment, a method of processing a substrate having a
transparent conductive oxide (TCO) disposed thereon includes exposing the
substrate to a first cleaning solution comprising hydrogen peroxide and
ammonium citrate to remove any organic contaminants from the TCO. The
substrate is then exposed to a second cleaning solution, different than
the first cleaning solution, and having a pH within a range from about 6
to about 7. The second cleaning solution is agitated to remove any
contaminants from the TCO. Each of the cleaning solutions is selected to
provide efficient removal of a variety of contaminants. A
silicon-containing film is then deposited on the transparent conductive
oxide.

[0009] In another embodiment, a method of processing a substrate having a
TCO disposed thereon comprises exposing the substrate to a first cleaning
solution comprising hydrogen peroxide. The first cleaning solution is
agitated, and then the substrate is rinsed with deionized water. The
substrate is then exposed to a second cleaning solution comprising
ammonium citrate or ammonium acetate, and then a silicon-containing film
is deposited on the transparent conductive oxide.

[0010] In another embodiment, a method of processing a substrate having a
TCO disposed thereon comprises exposing the substrate to a first cleaning
solution having a pH within a range from about 6 to about 7. The first
cleaning solution is agitated, and then substrate is rinsed with
deionized water. The substrate is then exposed to a second cleaning
solution comprising hydrogen peroxide and rinsed with deionized water. A
silicon-containing film is then deposited on the transparent conductive
oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] So that the manner in which the above recited features of the
present invention can be understood in detail, a more particular
description of the invention, briefly summarized above, may be had by
reference to embodiments, some of which are illustrated in the appended
drawings. It is to be noted, however, that the appended drawings
illustrate only typical embodiments of this invention and are therefore
not to be considered limiting of its scope, for the invention may admit
to other equally effective embodiments.

[0012] FIG. 1A is a schematic drawing of a single bath washer which may be
used to clean a substrate surface.

[0013] FIG. 1B is a schematic drawing of a dual bath washer which may be
used to clean a substrate surface.

[0014]FIG. 2A is a flow diagram illustrating one embodiment of processing
a substrate.

[0015]FIG. 2B is a flow diagram illustrating another embodiment of
processing a substrate.

[0016] To facilitate understanding, identical reference numerals have been
used, where possible, to designate identical elements that are common to
the figures. It is contemplated that elements disclosed in one embodiment
may be beneficially utilized on other embodiments without specific
recitation.

DETAILED DESCRIPTION

[0017] Embodiments of the present invention generally relate to methods
for cleaning a substrate prior to a deposition process. The methods
generally include exposing a substrate to multiple cleaning solutions to
remove contaminants from a surface of a substrate. The multiple solutions
generally have different compositions, and each of the solutions contain
one or more additives selected to remove a variety of contaminants.
Mechanical agitation may also be utilized to remove contaminants from the
surface of a substrate. After cleaning a substrate, a material may be
deposited on the substrate surface.

[0018] Methods described herein are beneficial for processing substrates
to be used in photovoltaic modules. Suitable apparatus for performing
methods herein include the one or more substrate cleaners of the
SunFab® thin film solar production line available from Applied
Materials, Inc. of Santa Clara, Calif. An example of a solar production
line is further described in U.S. patent application Ser. No. 12/202,199,
filed Aug. 29, 2008, which is herein incorporated by reference. It is
contemplated that other commercially available processing lines or stand
alone substrate washers may also benefit from methods disclosed herein.

[0019] FIG. 1A is a schematic drawing of a single bath washer 100 which
may be used to clean a substrate surface. Washer 100 comprises a cleaning
solution 104 disposed in wash tank 102. Coupled to the side of the wash
tank 102 is an optional ultrasonic mixer 106 to apply ultrasonic or
mechanical agitation to the cleaning solution 104. A substrate 108 may be
disposed in the cleaning solution 104 to have contaminants 110 removed
therefrom. Substrate 108 generally has a TCO layer disposed thereon. For
example, substrate 108 may have a layer of tin-doped indium oxide,
aluminum-doped zinc oxide, or indium-doped cadmium oxide disposed
thereon.

[0020] A sprayer 112 may provide pressurized fluid to remove contaminants
from the surface of the substrate 108. Alternatively, the pressurized
fluid may be used to remove cleaning solution from the surface of the
substrate 108 after removing substrate 108 from the cleaning solution
104. The pressurized fluid supplied from the sprayer 112 may be the same
fluid as in cleaning solution 104 or may be deionized water. In the
embodiment where sprayer 112 is adapted to provide a rinse solution to
the surface of substrate 108, such as deionized water, sprayer 112 may be
positioned in a location other than above cleaning solution 104. Thus, a
rinse solution could be provided to the surface of substrate 108, and the
concentration of the cleaning solution 104 would not be affected by the
rinse solution provided by sprayer 112. In one embodiment, multiple
washers 100 may be positioned successively to clean a plurality of
substrates 108 using an overhead track configuration. Alternatively, a
rinse station having a sprayer adapted to provide a rinse solution may be
positioned subsequent to a single bath washer.

[0021] FIG. 1B is a schematic drawing of a dual bath washer 101 which may
be used to clean a substrate surface. Dual bath washer 101 has a wash
tank 122 which is adapted to hold a first cleaning solution 124 and a
second cleaning solution 125. A cleaning solution divider 127 is
positioned between the first cleaning solution 124 and the second
cleaning solution 125 to prevent mixing of the cleaning solutions.
Substrates 128 and 129 may be positioned in the first cleaning solution
124 and the second cleaning solution 125, respectively, to remove
contaminants present on surfaces of substrates 128 and 129. A first
sprayer 121 is positioned above the first cleaning solution 124, while a
second sprayer 123 is positioned above a second cleaning solution 125.
First sprayer 121 and second sprayer 123 may be used to provide a pre- or
post-rinse solution such as deionized water, or may be used to provide
pressured cleaning solution to the surface of the substrate 108. In the
embodiment where sprayers 121 and 123 are adapted to provide a rinse
solution to the surface of substrates 128 and 129, sprayers 121 and 123
may be positioned in a location other than above cleaning solutions 124
and 125. Thus, a rinse solution could be provided to the surface of
substrates 128 and 129, and the concentration of the cleaning solutions
124 and 125 would not be affected by the rinse solution provided by
sprayers 121 and 123. Alternatively, substrates 128 and 129 may be rinsed
in an intermittent rinse bath (not shown) between exposure to the first
cleaning solution 124 and the second cleaning solution 125.

[0022] Generally, substrates 128 and 129 are sequentially cleaned by first
exposing substrate 128 or 129 to the first cleaning solution 124, and
then exposing the substrate 128 or 129 to the second cleaning solution
125. However, other cleaning configurations are contemplated. In
embodiments where substrates 128 and 129 are sequentially or
consecutively cleaned, the first cleaning solution 124 and the second
cleaning solution 125 generally have different compositions. In another
embodiment, two dual bath washers 101 may be consecutively positioned. In
such an embodiment, the first bath of each dual bath washer 101 may be a
cleaning solution, while the second bath of each dual bath washer 101 may
be a rinse solution. Therefore, during a substrate cleaning processing, a
substrate would be exposed to a cleaning solution, a rinse solution, a
cleaning solution, and then a final rinse solution, respectively.

[0023] Referring to FIG. 1B, the first cleaning solution 124 and the
second cleaning solution 125 may be selected to remove specific types of
contaminants from the surface of substrates 128 and 129. Contaminants are
generally introduced to the surface of substrates 128 and 129 when
substrates 128 and 129 are exposed to the ambient environment prior to or
between processes. Contaminants may also be introduced to the surface of
substrates 128 and 129 during previous substrate processing steps.
Different types of contaminants can be targeted and removed using
different compositions of cleaning solution. The composition of the first
cleaning solution 124 and the second cleaning solution 125 generally
include an additive added to a bath of deionized water. Exemplary
additives include chelating agents, oxidizing agents, surfactants, and
pH-adjusting solutions. Alternatively, the first cleaning solution 124
and the second cleaning solution 125 may comprise an undiluted solution,
or a solvent other than deionized water. The first cleaning solution 124
and/or the second cleaning solution 125 may contain any combination of
cleaning additives to remove desired contaminants, so long as the
combined cleaning additives do not substantially affect the cleaning
ability of other additives present in the first or second cleaning
solutions.

[0024] Embodiments described herein will generally refer to a two bath or
two solution cleaning processes. Either of the first cleaning solution or
the second cleaning solution may include any of the cleaning additives
discussed herein for removal of contaminants, unless specifically stated
otherwise. The term "and/or" is used to clarify that the cleaning
additives discussed in the following description may be present in the
first cleaning solution, or the second cleaning solution, or both the
first and second cleaning solutions when necessary to effect a desired
level of cleaning. While the first cleaning solution and the second
cleaning solution generally have different compositions, it is
contemplated that the solutions may have the same compositions when
required for sufficient substrate cleaning. In some embodiments, the
second cleaning solution may be deionized water used to rinse a substrate
previously exposed to a cleaning solution.

[0025] In one embodiment, the first cleaning solution 124 and/or the
second cleaning solution 125 may be used to remove organic contaminants
from the surface of a substrate. The presence of organic contaminants on
TCO may prevent the formation of an effective TCO/silicon interface when
a silicon layer is subsequently formed on the TCO. The reduced quality of
the TCO/silicon interface can result in reduced photovoltaic cell
efficiency. Therefore, it is desirable to remove organic contaminants.
Organic contaminants can be sufficiently removed from a substrate using
oxidizing agents and/or surfactants. Suitable oxidizing agents include
peroxides such as hydrogen peroxide, sulfuric acid, or nitric acid.
Oxidizing agents may be used in a concentration from about 0.1 percent to
about 10 percent by volume in either or both of the first cleaning
solution 124 or the second cleaning solution 125. For example, the first
cleaning solution 124 may contain about 0.1 percent to about 5 percent
hydrogen peroxide by volume in deionized water. Oxidants such as
peroxides efficiently remove organic contaminants from the surface of TCO
by oxidizing the contaminant.

[0026] Surfactants can also be used to remove organic contaminants from a
substrate surface. Surfactants remove organic contaminants by forming
micelles around the organic contaminants which assist in moving the
contaminant from the surface of the substrate to the cleaning solution.
Suitable surfactants include sulfates, sulfonates, carboxylates, and
polyethylene oxides. Surfactants may be used in the first cleaning
solution 124 and/or the second cleaning solution 125 in a concentration
between about 0.001 percent and about 5 percent by volume.

[0027] In another embodiment, the first cleaning solution 124 and/or the
second cleaning solution 125 may be used to remove metal ion contaminants
from the surface of a substrate. Metal ions which are present on a
substrate before a silicon film is deposited thereon may contaminate the
subsequently deposited film, leading to reduced photovoltaic cell
efficiency. Metal ions can be removed from a surface of a substrate using
chemicals with chelating or complexing capabilities. Chelating agents
remove metal ions from a surface of a substrate by forming soluble,
complex molecules with the metal ions. The soluble complex molecules
reduce the occurrence of the metal ions reacting with other elements and
assist in moving the metal ions into the cleaning solution. Suitable
chelating agents include ammonium citrate and ammonium acetate,
ethylenediaminetetraacetic acid (EDTA), and phosphonates. Chelating
agents may be present in the first cleaning solution 124 and/or the
second cleaning solution 125 at a concentration of about 0.1 percent to
about 10 percent by volume, for example, about 3 percent by volume.

[0028] The first cleaning solution 124 and/or the second cleaning solution
125 may also be used to remove particles and particulate contaminants
from the surface of a substrate. Particle contaminants may include
organic matter or inorganic matter. For example, particle contaminants
may include TCO residue generated in a texturing or laser scribing
processing. Particle contaminants can be removed from the surface of a
substrate by adjusting the pH of the first cleaning solution 124 and/or
the second cleaning solution 125 in conjunction with or as an alternative
to agitation. The pH of the first cleaning solution 124 or the second
cleaning solution 125 is preferably adjusted to within a range from about
4 to about 7, for example about 4 to about 5 or about 6 to about 7. The
pH of the first cleaning solution 124 or the second cleaning solution 125
causes the particle contaminants present on the substrate to have
approximately the same surface charge as the substrate itself. When the
particle contaminants and the substrate have approximately the same
charge, the Van der Waals interaction between the two are reduced,
allowing the particle contaminants to more easily be washed away or
dissolved in cleaning solution.

[0029] The pH of the first cleaning solution or the second cleaning
solution can be adjusted by adding a pH-adjusting solution to the first
cleaning solution 124 and/or the second cleaning solution 125. A
pH-adjusting solution is any solution which can be used to adjust the pH
of the first or second cleaning solutions, such as an acid or a base.
Exemplary bases include potassium hydroxide and ammonium hydroxide. An
exemplary acid is hydrochloric acid. Any suitable acid or base which does
not adversely affect the substrate or the cleaning ability of other
additives present in the first cleaning solution 124 or the second
cleaning solution 125 may be used. The molarity of the pH-adjusting
solution can be chosen based upon the desired amount of adjustment of the
cleaning solution pH.

[0030] In addition to adjusting the pH of the first cleaning solution 124
or the second cleaning solution 125, particle contaminants may also be
removed with the assistance of ultrasonic or megasonic agitation. The
ultrasonic or megasonic agitation may be effected by an ultrasonic or
megasonic mixer. Agitation provided by the ultrasonic or megasonic mixer
assists in overcoming Van der Waals interactions so that particle
contaminants can more easily be separated form a substrate surface. An
ultrasonic mixer may be immersed within a cleaning solution bath to
provide ultrasonic agitation, or may be coupled to the side of a washer
and adapted to provide ultra sonic agitation to a cleaning solution bath.
In the embodiment of FIG. 1A, an ultrasonic mixer is coupled to the side
of wash tank 102 and adapted to agitate cleaning solution 104.

[0031] Prior to or subsequent to exposing a substrate to one or more
cleaning solutions 124 or 125, the substrate may also be exposed to an
inert gas plasma to remove organic contaminants from a surface of the
substrate. For example, a substrate may be exposed to argon plasma for
about 30 seconds to about 180 seconds to remove organic contaminants from
a surface of a substrate. In one embodiment, a substrate is exposed to an
inert gas plasma for about 120 seconds. A substrate may be exposed to an
inert gas plasma before the first cleaning solution 124, after the second
cleaning solution 125, or intermittently between the first cleaning
solution 124 and the second cleaning solution 125.

[0032]FIG. 2A is a flow diagram illustrating one embodiment of processing
a substrate. In flow diagram 230a, a substrate is cleaned in step 231
using a first cleaning solution. The first cleaning solution may be
disposed in a single bath washer as described in FIG. 1A, or may be
disposed in the first bath of a dual bath washer as described in FIG. 1B.
The first cleaning solution of step 231 may contain one or more of any of
the cleaning additives described herein, so long as the combination of
additives does not significantly impair the cleaning ability of other
components present in the cleaning solution. For example, the first
cleaning solution may be used to remove one or more of inorganic
particles, metal ions, or organic contaminants. Generally, step 231 is
subsequent to a TCO texturing process, and may be subsequent to a TCO
patterning or scribing process. It is preferred, although not necessary,
that step 231 follows the texturing and patterning processes in order to
remove the contaminants generated thereby.

[0033] In step 233, a substrate is transferred from the first cleaning
solution to a second cleaning solution. The second cleaning solution may
be a bath disposed in a second single bath washer as described in FIG.
1A, or may be a second bath disposed in a dual bath washer as described
in FIG. 1B. Alternatively, the first cleaning solution may be drained
form a single bath washer and replaced with the second cleaning solution.
The second cleaning solution of step 233 is formulated to target and
remove specific contaminants not removed in step 231. For example, if
inorganic particle contaminants remain on a substrate surface after
exposure to the first cleaning solution, then the second cleaning
solution may contain an agitated pH-adjusted solution. As described
above, pH-adjusted solutions in combination with mechanical agitation
effectively remove inorganic particle contaminants present on the surface
of a substrate. In step 235, a post-clean process may be performed. In
one embodiment, the post-clean process is a film deposition process, such
as a plasma-enhanced chemical vapor deposition of amorphous silicon.

[0034]FIG. 2B is a flow diagram illustrating another embodiment of
processing a substrate. In flow diagram 230b, a substrate is cleaned in
step 232 using a first cleaning solution. The first cleaning solution may
be disposed in a single bath washer as described in FIG. 1A, or may be
the first bath of a dual bath washer as described in FIG. 1B. Generally,
step 232 is subsequent to a TCO texturing process. The first cleaning
solution of step 232 may contain any combination of the cleaning
components or additives described herein. For example, the first cleaning
solution may be used to remove one or more of inorganic particles, metal
ions, or organic contaminants. Therefore, the first cleaning solution
would contain a cleaning additive sufficient to remove the selected or
desired contaminant, as described above.

[0035] In step 234, an optional intermittent substrate process may be
performed. The optional intermittent substrate process may be a laser
scribe or laser patterning process. However, since the laser scribe or
laser patterning process may generate residue, such as TCO residue, it
may be necessary to remove this residue form the substrate after the
intermittent substrate process. In step 236, a substrate may be cleaned
with a second cleaning solution. If inorganic particles are located on a
surface of the substrate, such as TCO residue generated in step 234, then
it is desirable to tailor the second cleaning solution to remove the
inorganic particles. For example, the second cleaning solution may be an
ultrasonically agitated pH-adjusted solution having a pH of about 5. In
step 238, a post-clean process, such as a film deposition, may be
performed on the substrate. It is preferred that the post-clean process
occurs soon after step 236 to avoid the accumulation of extra
contaminants on the substrate surface due to extended exposure to the
surrounding environment.

[0036] Table 1 illustrates the efficiency of photovoltaic cells
manufactured from substrates cleaned by methods described herein.
Referring to Example 1 of Table 1, a first cleaning solution containing
deionized water with 3 percent hydrogen peroxide by volume is used to
clean three 30 centimeter by 30 centimeter substrates. The three
substrates were then rinsed with a second solution of deionized water.
After forming photovoltaic cells with the cleaned substrates, the
photovoltaic cells had a mean conversion efficiency of 8.32 percent. The
individual photovoltaic cells had efficiencies of 8.46 percent, 8.41
percent and 8.09 percent, respectively. The mean conversion efficiency of
Example 1 represents about a 24 percent increase in conversion efficiency
when compared to standard cleaning techniques.

[0037] In Example 2, a two solution process was used to clean three 30
centimeter by 30 centimeter substrates. The first solution contained one
percent ammonium citrate by volume in deionized water. A second solution
contained about 96 percent isopropyl alcohol by volume in deionized
water. The three substrates cleaned in Example 2 were used to form
photovoltaic cells which had a mean conversion efficiency of 8.22 percent
after processing. The individual photovoltaic cells had efficiencies of
7.85 percent, 8.29 percent, and 8.51 percent, respectively. As an
alternative to the second solution, isopropyl alcohol wipes containing
substantially pure isopropyl alcohol could be used to clean the substrate
surfaces.

[0038] In example 3, three 30 centimeter by 30 centimeter substrates were
cleaned with a first cleaning solution containing one percent ammonium
citrate by volume in deionized water. The substrates were then cleaned
with a second cleaning solution containing 96 percent isopropyl alcohol
by volume, and then exposed to argon plasma for about 120 seconds. The
three substrates processed in Example 3 were used to form photovoltaic
cells which had a mean conversion efficiency of 7.77 percent. The
individual photovoltaic cells had efficiencies of 7.14, 8.30, and 7.86,
respectively.

[0039] In Example 4, four substrates were cleaned in a first cleaning
solution of one percent ammonium carbonate by volume. The four substrates
were then rinsed with a second solution of deionized water. The four
substrates of Example 4 were used to form photovoltaic cells which had a
mean conversion efficiency of 7.17 percent. The individual photovoltaic
cells had efficiencies of 7.19 percent, 7.34 percent, 7.74 percent, and
6.42 percent, respectively.

[0040] In Example 5, three 30 centimeter by 30 centimeter substrates were
cleaned with a first cleaning solution of one percent ammonium citrate by
volume. The three substrates were then rinsed with a second solution of
deionized water. The three substrates cleaned with the cleaning solution
of one percent ammonium citrate and subsequently rinsed with deionized
water were used to form photovoltaic cells which had a mean conversion
efficiency of 7.84 percent. The individual photovoltaic cells had
efficiencies of 7.53 percent, 8.33 percent, and 7.65 percent,
respectively. In Example 6, a first cleaning solution comprising
potassium hydroxide, sodium silicate, and sodium perchlorate with a pH of
about 10-12 was used to clean four substrates. The four substrates were
then rinsed with deionized water. The four substrates of Example 6 were
used to form photovoltaic cells which had a mean conversion efficiency of
6.72 percent. The individual photovoltaic cells had efficiencies of 6.42
percent, 5.87 percent, 7.86 percent, and 6.72 percent, respectively.

[0041] In Example 7, three 30 centimeter by 30 centimeter substrates were
cleaned with a first cleaning solution of about 96 percent isopropyl
alcohol by volume. The three substrates were then rinsed with deionized
water. The three substrates of Example 7 were used to form photovoltaic
cells having a mean conversion efficiency of 8.32 percent. The individual
photovoltaic cells had efficiencies of 8.57 percent, 7.83 percent, and
8.55 percent. Having approximately 15 percent light induced degradation,
the average final stabilized conversion efficiency of a single junction
module manufactured with the substrates of Example 7 was about 7 percent.
With 20 percent light induced degradation, the average final stabilized
conversion efficiency of a single junction module manufactured with the
substrates of Example 7 was about 6.6 percent.

[0042] Table 2 illustrates some embodiments for cleaning substrates. The
embodiments shown in Table 2 are provided to illustrate some of the
possible combinations and compositions of first and second cleaning
solutions for use in a dual bath washer. The cleaning solutions discussed
in the embodiments of Table 2 may be provided in the concentrations
detailed above. Other combinations and compositions of first and second
cleaning solutions are possible, and may be dictated by the types of
contaminants desired to be removed.

[0043] In Example 8, a first washer has a cleaning solution comprising a
pH-adjusted solution. The cleaning solution of the first washer is
agitated to remove organic and inorganic particles. A cleaning solution
in a second washer comprises peroxide to remove remaining organic
contaminants. Although not shown, Washer 1 or Washer 2 may additionally
contain ammonium citrate to remove metal ions. In Example 9, Washer 1
contains peroxide to remove organic material, and Washer 2 contains an
agitated pH-adjusted solution to remove organic and inorganic particles.
Additionally, although not shown, a chelating agent may be added to
Washer 1 or Washer 2 to remove metal ions form a substrate surface.

[0045] As described above, the use of targeted chemistries to remove
specific types of contaminants from the surface of substrates allows for
more effective cleaning of substrate surfaces. Cleaner substrate surfaces
allow for higher conversion efficiencies to be obtained from processed
photovoltaic modules. Additionally, the use of a multiple bath system
allows for individual baths to be formulated to remove targeted
contaminants. The specifically formulated cleaning solutions allow for
more effective removal of contaminants since the cleaning solutions can
be designed to remove specific contaminants. The cleaner surfaces and
higher conversion efficiencies of the processed substrates reduce the
cost per unit energy of manufactured photovoltaic modules.

[0046] While the foregoing is directed to embodiments of the present
invention, other and further embodiments of the invention may be devised
without departing from the basic scope thereof, and the scope thereof is
determined by the claims that follow.

Patent applications by Adam Brand, Palo Alto, CA US

Patent applications by Dapeng Wang, Santa Clara, CA US

Patent applications by Liming Zhang, San Jose, CA US

Patent applications by Renhe Jia, Berkeley, CA US

Patent applications by Tzay-Fa Su, San Jose, CA US

Patent applications by Vijay Parihar, Fremont, CA US

Patent applications by Applied Materials, Inc.

Patent applications in class Including application of electrical radiant or wave energy to work

Patent applications in all subclasses Including application of electrical radiant or wave energy to work